1. Field of the Invention
This invention relates to pneumatic tires for vehicles and specifically to a tire designed to run while deflated or underinflated.
2. Description of the Prior Art
A basic problem with all pneumatic tires is that they occasionally become underinflated or completely deflated and when this occurs, the tire must be changed and a spare tire put on. In some cases a blowout can cause the vehicle to go out of control.
A tire which can be run flat has for some time been a desirable objective in the tire-making art. If a tire could be run flat for an appreciable distance, the driver could run on the flat tire until a replacement tire was obtained or the tire repaired. This would eliminate changing tires on the road and dependence on a spare. A driver could also run on the suddenly deflated tire until a safe place to stop the car is found, thus avoiding sudden stopping on crowded streets and highways.
There are many problems associated with running a conventional tire flat. A flat tire is unstable, making steering difficult. The lack of inflation pressure causes the tire beads to unseat, and eventually the tire may come off the wheel rim. In addition, riding with a flat tire can be an uncomfortable experience since there is practically no cushion between the wheel rim and the road surface.
A number of designs have been proposed to increase the stability and rideability of the tire when deflated or flat. Some of these proposals, such as U.S. Pat. Nos. 3,394,751 and 3,421,566 relate to movable sidewalls so that the tire tread force is communicated directly to the rim. Other proposals, such as U.S. Pat. Nos. 2,040,645; 3,392,722 and 3,610,308 have special units in the interior of the tire.
A problem that occurs in the run-flat tire and that has not been solved by these proposals is maintaining the seat of the tire beads in the rim. When a conventional pneumatic tire is inflated, the inflation pressure forces the beads against the inside edges or flanges and the bead seat of the rim, thus keeping the beads seated against the rim, thereby maintaining tire stability. Upon deflation, the force against the beads is gone, and the beads tend to unseat. This phenomenon prevents the tire from being run while flat for any appreciable distance. At the present time, if a standard pneumatic tire is run flat for some distance, the beads may unseat and subsequently demount from the rim.
An additional problem generated by tires run flat is the friction which develops from the upper and lower portions of the deflated sidewall rubbing against each other. The friction produces excess heat and causes the sidewalls to wear excessively. To reduce this friction, the inclusion of either liquid or solid lubricants on the tire interiors has been proposed. U.S. Pat. No. 2,040,645, for instance, suggests a graphite lubricant, U.S. Pat. No. 3,610,308 mentions the use of liquid silicone, and U.S. Pat. Nos. 3,739,829 and 3,850,217 describe the use of polyalkylene glycols, glycerol, propylene glycol, silicone and other lubricants.